In general, position encoders are devices which determine the instantaneous physical position of a movable object with respect to a fixed reference point, and translate such position information into a form that can be utilized by a processing, analytical or other type, tool. A position encoder typically transforms position information into an electrical signal, and provides the electrical signal to an analog or digital signal processor. Position encoders may determine angular position, as in the case of a rotatable shaft or toroidal structure (e.g., an automobile tire), or they may determine linear position, as in the case of a slidable control actuator. An ideal position encoder produces an output signal that is a linear function of the position of the movable object. An improved position encoder is described and claimed in my copending application, U.S. patent application Ser. No. 09/315,205, filed contemporaneously herewith, and assigned to the present assignee (Attorney Docket No. ADL-091). Instantaneous position information, sampled over time, may be used to determine higher derivatives of position such as velocity and acceleration.
Typical position encoders operate either mechanically, optically or magnetically. A mechanical encoder relies upon physical contact with the movable object; actuators on the movable object physically interact with an electro-mechanical transducer to produce an electrical signal. An optical encoder receives light reflected from illuminated markings associated with the movable object and translates variations in the received light into an electrical signal. Magnetic encoders typically utilize either fluxgate sensors or Hall effect sensors. A fluxgate sensor magnetic encoder uses fluxgate sensors to detect the magnetic field generated by magnetic elements attached to the movable object, and translates aspects of the magnetic field such as magnitude and direction into an electrical signal corresponding to the position of the object. A Hall effect sensor magnetic encoder translates the Hall effect of a magnetic field on a current carrying conductor to produce a signal corresponding to the position of the object. Fluxgate position encoders are several orders of magnitude more sensitive than Hall effect position encoders and are thus preferred in applications where it may be difficult to have the sensors in close proximity of the magnetic element producing the magnetic field. For example, in an application to determine the angular position of an automobile tire, the close proximity a Hall effect sensor requires is difficult to maintain because of the harsh environment created by road dirt, oil, grease, ice and snow.
A fluxgate sensor includes one or more turns of an electrical conductor wound about a core, which is disposed along a sensing axis. The core may be any material, including air, although high permeability materials such as iron or nickel are usually preferred. An external driving circuit alternately drives the sensor into saturation in one polarity and then into the opposite polarity. The external driving circuit drives current through the windings in one direction until the core saturates. Once the core saturates, the driving circuit reverses current in the windings until the core saturates in the opposite polarity. In the absence of an external magnetic field, the amount of time the driving circuit drives current in each direction is the same; i.e., the current waveform through the windings as a function of time is symmetrical. The presence of an external magnetic field "helps" (i.e., enhances) the saturation of the core in one polarity, while the external magnetic field impedes the saturation of the core in the opposite polarity. Thus, in the presence of an external magnetic field, the waveform of the current through the windings as a function of time is asymmetrical, since saturation occurs more quickly for the polarity of the saturation enhanced by the external field. The amount of asymmetry may be used to determine characteristics of the external magnetic field, such as magnitude and direction.
The amount of current necessary to drive an inductor coil into saturation varies with the number of windings, the core material, etc. However, for a typical flux gate sensor, the amount of current necessary to drive the sensor into saturation will be on the order of tens of mA. Since this current is entirely supplied by the driver circuit, the input power requirements of such a driver circuit are defined by the saturation current of the fluxgate sensor. For example, U.S. Pat. No. 4,859,944, "Single Winding Magnetometer With Oscillator Duty Cycle Measurement," invented by Spencer L. Webb, discloses a driver circuit which essentially alternately connects a positive voltage source and a negative voltage source across an inductor coil. A few tens of mA is not generally considered a large amount of current. However, in low power applications, such as portable electronic systems which operate from a battery power source, current requirement goals are typically in the micro-ampere range.
It is an object of this invention to provide a position encoder that substantially overcomes or reduces the aforementioned disadvantages while providing other advantages which will be evident hereinafter.